[clang][dataflow] Disallow setting properties on `RecordValue`s. (#76042)

Instead, synthetic fields should now be used for the same purpose. These
have a
number of advantages, as described in
https://github.com/llvm/llvm-project/pull/73860, and longer-term, we
want to
eliminate `RecordValue` entirely.

As `RecordValue`s cannot have properties any more, I have replaced the
`OptionalIntAnalysis` with an equivalent analysis that tracks nullness
of
pointers (instead of whether an optional has a value). This serves the
same
purpose, namely to check whether the framework applies a custom
`merge()`
operation to widen properties.